Long and Short Isoforms of the Human Cytomegalovirus UL138 Protein Silence IE Transcription and Promote Latency

Abstract

The UL133-138 locus present in clinical strains of human cytomegalovirus (HCMV) encodes proteins required for latency and reactivation in CD34؉ hematopoietic progenitor cells and virion maturation in endothelial cells. The encoded proteins form multiple homo-and hetero-interactions and localize within secretory membranes. One of these genes, UL136 gene, is expressed as at least five different protein isoforms with overlapping and unique functions. Here we show that another gene from this locus, the UL138 gene, also generates more than one protein isoform. A long form of UL138 (pUL138-L) initiates translation from codon 1, possesses an amino-terminal signal sequence, and is a type one integral membrane protein. Here we identify a short protein isoform (pUL138-S) initiating from codon 16 that displays a subcellular localization similar to that of pUL138-L. Reporter, short-term transcription, and long-term virus production assays revealed that both pUL138-L and pUL138-S are able to suppress major immediate early (IE) gene transcription and the generation of infectious virions in cells in which HCMV latency is studied. The long form appears to be more potent at silencing IE transcription shortly after infection, while the short form seems more potent at restricting progeny virion production at later times, indicating that both isoforms of UL138 likely cooperate to promote HCMV latency. IMPORTANCELatency allows herpesviruses to persist for the lives of their hosts in the face of effective immune control measures for productively infected cells. Controlling latent reservoirs is an attractive antiviral approach complicated by knowledge deficits for how latently infected cells are established, maintained, and reactivated. This is especially true for betaherpesviruses. The functional consequences of HCMV UL138 protein expression during latency include repression of viral IE1 transcription and suppression of virus replication. Here we show that short and long isoforms of UL138 exist and can themselves support latency but may do so in temporally distinct manners. Understanding the complexity of gene expression and its impact on latency is important for considering potential antivirals targeting latent reservoirs. Human cytomegalovirus (HCMV) is a betaherpesvirus that causes birth defects and disease in immunocompromised and immunosuppressed patients and has been associated with cancers, cardiovascular disease, and immune dysfunction (1-3). HCMV productively (lytically) infects differentiated cells, such as fibroblasts, endothelial cells, and epithelial cells, and latently infects incompletely differentiated cells of the myeloid lineage, such as monocytes and CD34 ϩ hematopoietic progenitor cells (HPC) (4, 5). Productive infection in multiple cell types within the human host facilitates viral dissemination, while latency ensures lifelong persistence despite an effective immune response against lyticphase antigens. Periodic reactivations of latent infections into the productive phase perpetuate both lifelong infecti...

The Loss of Binary: Pushing the Herpesvirus Latency Paradigm

“…The opposing regulation of EGFR by UL135 and UL138 may alter the cellular state, and, in turn, impact programs of viral gene expression contributing to virus-mediated control of latency and reactivation. Consistent with this notion, UL138 has also been shown to impede demethylation of histones associated with the viral genome required for expression of IE genes [97], suggesting that UL138 regulates signaling cascades which will ultimately impact the chromatin state of the viral genome. Therefore, control over a homeostatic regulator like EGFR may effectively allow the virus to hardwire itself into the host cell to sense and respond to changes in the state of the cell.…”

The Membrane-Spanning Peptide and Acidic Cluster Dileucine Sorting Motif of UL138 Are Required To Downregulate MRP1 Drug Transporter Function in Human Cytomegalovirus-Infected Cells

“…The putative amino-terminal transmembrane sequences of UL138 are largely absent in a naturally occurring isomer that initiates translation from the internal methionine-16 codon (29), expressing an N-terminally truncated short isoform. It is not known which isoform, i.e., the (full-length) long isoform or the short isoform, is more important during clinical infection.…”

“…It is not known which isoform, i.e., the (full-length) long isoform or the short isoform, is more important during clinical infection. In vitro, the long isoform (expressed from methionine-1) is the major species; however, the short isoform is expressed and appears to play a role in the establishment and maintenance of latency (29). Latency establishment and maintenance in vitro are most efficient when the two isoforms of UL138 are naturally simultaneously expressed from the wild-type (WT) gene (29).…”

“…In vitro, the long isoform (expressed from methionine-1) is the major species; however, the short isoform is expressed and appears to play a role in the establishment and maintenance of latency (29). Latency establishment and maintenance in vitro are most efficient when the two isoforms of UL138 are naturally simultaneously expressed from the wild-type (WT) gene (29). The primary subcellular location of both UL138 isoforms is the Golgi apparatus (28,29).…”

“…Latency establishment and maintenance in vitro are most efficient when the two isoforms of UL138 are naturally simultaneously expressed from the wild-type (WT) gene (29). The primary subcellular location of both UL138 isoforms is the Golgi apparatus (28,29).…”